Implantable medical leads are used to provide electrical stimulation from a pulse generator to a target site within a body of a patient. The lead includes electrical conductors that extend from a proximal end that is coupled to the pulse generator to a distal end. The conductors carry stimulation signals to electrodes on the distal end that are positioned at the target site and deliver the stimulation signals to the tissue.
The presence of the lead presents a risk if the patient undergoes a magnetic resonance imaging (MRI) scan. Radio frequency (RF) energy that is present during the MRI scan may couple to the conductor(s) within the lead which results in electrical current on the conductor that can cause potentially dangerous heating of tissue nearby the electrode. This is especially problematic for neurostimulation leads where the electrode is placed in very sensitive neurological tissue such as within the brain or spine.
Various techniques have been devised to try to lessen the current being induced onto the conductor by the RF energy to thereby lessen the amount of heating at the electrode. One technique is to include a conductive RF shield that surrounds the conductor. The RF energy is largely blocked from reaching the conductor and the induced current and tissue heating are reduced.
The conductor is typically located in a lumen of the lead while the shield may be present outside of the lumen, typically in a polymer jacket. Over time, body fluids infiltrate the polymer jacket of the lead and reach the lumen which fills with the fluid. Thus, a significant amount of body fluid could be present between the shield and the conductor being shielded. Because the body fluid presents a high dielectric constant, capacitive coupling may occur to some degree between the shield and the conductor which could result in some of the RF energy being transferred to the conductor.